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Abstract
Apatite is a common magmatic accessory in the intrusive rocks hosting the giant ~1590 Ma Olympic Dam (OD) iron-oxide copper gold (IOCG) ore system, South Australia. Moreover, hydrothermal apatite is a locally abundant mineral throughout the altered and mineralized rocks within and enclosing the deposit. Based on compositional data for zoned apatite, we evaluate whether changes in the morphology and the rare earth element and Y (REY) chemistry of apatite can be used to constrain the fluid evolution from early to late hydrothermal stages at OD. The ~1.6 Ga Roxby Downs granite (RDG), host to the OD deposit, contains apatite as a magmatic accessory, locally in the high concentrations associated with mafic enclaves. Magmatic apatite commonly contains REY-poor cores and REY-enriched margins. The cores display a light rare earth element (LREE)-enriched chondrite-normalized fractionation pattern with a strong negative Eu anomaly. In contrast, later hydrothermal apatite, confined to samples where magmatic apatite has been obliterated due to advanced hematite-sericite alteration, displays a conspicuous, convex, middle rare earth element (MREE)-enriched pattern with a weak negative Eu anomaly. Such grains contain abundant inclusions of florencite and sericite. Within high-grade bornite ores from the deposit, apatite displays an extremely highly MREE-enriched chondrite-normalized fractionation trend with a positive Eu anomaly. Concentrations of U and Th in apatite mimic the behaviour of ∑REY and are richest in magmatic apatite hosted by RDG and the hydrothermal rims surrounding them. The shift from characteristic LREE-enriched magmatic and early hydrothermal apatite to later hydrothermal apatite displaying marked MREE-enriched trends (with lower U, Th, Pb and ∑REY concentrations) reflects the magmatic to hydrothermal transition. Additionally, the strong positive Eu anomaly in the MREE-enriched trends of apatite in high-grade bornite ores are attributable to alkaline fluid conditions.
Highlights
The concentrations of trace elements within hydrothermal minerals and their variation can provide valuable information on fluid parameters and the conditions of ore deposition if the studied assemblages are well constrained with respect to their paragenetic position [1]
We aim to show that rare earth element and Y (REY) signatures in apatite can trace the magmatic to hydrothermal transition in granite-hosted iron-oxide copper gold (IOCG)
Apatite displays a mottled texture (Figure 2a) due to the presence of sub-micron to nanometre-scale inclusions of pyrrhotite and fluorite, which were observed in abundance in parts of Roxby Downs granite (RDG) with numerous mafic enclaves
Summary
The concentrations of trace elements within hydrothermal minerals and their variation can provide valuable information on fluid parameters and the conditions of ore deposition if the studied assemblages are well constrained with respect to their paragenetic position [1]. Minerals 2017, 7, 135 chondrite-normalized REY fractionation patterns as geochemical tracers that can assist interpretation of hydrothermal systems. Variation in the magnitude of Eu anomalies across rock suites spanning entire metallogenic provinces have been used to infer variability or change in redox conditions in magmatic apatite [8]. The deposit is hosted within the Olympic Dam Breccia Complex (ODBC; [23]), a collective term used to describe the breccias, intrusive phases and minor bedded sediments which host the deposit. This is hosted within the Roxby Downs granite (RDG), a mediumto coarse-grained, undeformed, pink, two-feldspar granite, locally displaying rapakivi textures [19]. Contemporaneous with the aforementioned granitoids, and hosted within the ODBC, are a swarm of felsic to ultramafic dykes, including strongly altered olivine-phyric dolerites (dolerite dyke, DD) [20,25]
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